Analysis of 210Pb in sediment trap samples and sediments from the northern Arabian Sea: evidence for boundary scavenging

Abstract

The concentrations of 210Pb were measured in particles intercepted by sediment traps deployed at 1024 m and 3024 m in the northern Arabian Sea for a period of 1 year from November 1986 to October 1987. The 210Pb activity in the biweekly samples varied from 11 to 108 dpm g −1 at 1024 m and 58 dpm g −1 to 588 dpm g −1 at 3024 m. The mean annual 210Pb flux increased from 0.27 dpm cm −2 yr −1 in the 1024 m trap to 0.65 dpm cm −2 yr −1 in the 3024 m trap. The 210Pb flux exhibited a strong seasonal pattern associated with variations in the sediment mass flux and organic carbon (C org) flux except during early southwest monsoon in the 3024 m trap. This could be due to enhanced scavenging of 210Pb on inorganic phases consisting mostly of Fe–Mn oxides. 210Pb concentrations were dependent on particle composition, indicating that 210Pb scavenging from the water column is directly coupled to the type of settling particles. Organic matter does not appear to be the dominant carrier phase for 210Pb as shown by the poor 210Pb–C org correlation. The fraction of 210Pb removed from the ocean by settling particles F/ P ( 210Pb Flux / 210Pb production) cannot be used as a proxy to determine the C org flux in this region. The excess 210Pb inventories in 13 sediment cores in the Arabian Sea ranged from 11 to 211 dpm cm −2. Such highly variable spatial distribution of the excess 210Pb inventory reflects a non-uniform deposition of 210Pb in this region. The measured excess 210Pb inventories (124–211 dpm cm −2) for the sea floor sediments below the Western Arabian Sea Trap and from other locations in the vicinity of the sediment trap mooring lend support to boundary scavenging within the northwestern Arabian Sea as a sink for 210Pb.